The inventive concept relates generally to nonvolatile memory devices and, more particularly, to methods of operating nonvolatile memory devices.
Data is programmed to and read from the individual memory cells of a nonvolatile memory device in accordance with a plurality of threshold voltage distributions. That is, respective threshold voltage distributions are assigned a corresponding logic state.
Nonvolatile memory cells may be configured as single-level memory cells (SLC) capable of storing one bit of data per SLC, or as multi-level memory cells (MLC) capable of storing two or more bits per MLC. As the number of bits (“k”) stored by MLC increases, the number of corresponding program states (and associated threshold voltage distributions) increases according to the relationship “2k”. As a result the voltage level widths of respective threshold voltage distributions used to program MLC can become quite narrow. That is, given a maximum threshold voltage range for a MLC, this range must be ever more finely partitioned into coherent threshold voltage distributions, each respectively associated with a corresponding data state.
To better manage the programming, reading, erasing, and/or storing of data, the memory cell arrays of contemporary nonvolatile memory devices are divided in a number of memory blocks, where each memory block is further divided into a plurality of pages. This division of blocks and pages may be made in view of logical data definitions and/or the physical layout of memory cells in a memory cell array. Data is usually programmed/read according to page units while data is erased according to block units.
Circumstances often arise wherein some of the pages in a given block have been programmed, but other pages remain un-programmed (i.e., remain in an erased state). Under these conditions, one or more read operations may be directed to memory cells of the “programmed pages”. Unfortunately, under the stress or influence of executing such read operations, the threshold voltage distribution of the memory cells in the “un-programmed pages” may be undesirably shifted (or broadened). This effect is a type of read disturbance. In extreme cases of read disturbance, when a program operation is subsequently directed to one of the un-programmed pages—now having memory cells with shifted threshold voltage distribution(s) being interpreted as fail bits (e.g., erroneously programmed bits)—the program operation may fail.